Intracavity probes and interfaces therefor for use in obtaining images and spectra of intracavity structures using high field magnetic resonance systems

ABSTRACT

An intracavity probe for use with an MR system allows images or spectra of a region of interest within a cavity of a patient to be obtained. The probe includes a shaft, a balloon at one end thereof, and a coil loop within the balloon. The coil loop preferably includes two drive capacitors and a tuning capacitor, all of which in series. A junction node between the drive capacitors serves as a ground for electrically balancing the coil loop. Diametrically opposite the junction node, the tuning capacitor enables the coil loop to resonate at the operating frequency of the MR system. Across each drive capacitor is connected an output cable having an electrical length of S L +n(λ/4). The output cables terminate in a plug that is used to connect the coil loop to an interface device for the intracavity probe.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the national stage application for PCT/US2005/041912filed Nov. 15, 2005 that claims the benefit of U.S. ProvisionalApplication 60/628,166 filed Nov. 15, 2004 and is a continuation-in-partof PCT International Application No. PCT/US2003/007774 filed Mar. 13,2003. That provisional application has been assigned to the assignee ofthe invention disclosed below, and its teachings are incorporated intothis document by reference.

FIELD OF THE INVENTION

The present invention generally relates to systems and methods ofobtaining images and spectra of intracavity structures using magneticresonance (MR) systems. More particularly, the invention pertains to anintracavity probe capable of being inserted into various bodilyopenings, such as the rectum, vagina, mouth, etc., to obtain highresolution images of and spectroscopic results for regions of interesttherein. Even more particularly, the invention relates to interfacedevices designed to interface such an intracavity probe with 2.0 Teslato 5.0 Tesla MR systems to obtain such high resolution images andspectroscopic results for such regions of interest.

BRIEF DESCRIPTION OF RELATED ART

The following background information is provided to assist the reader tounderstand the invention disclosed below and the environment in which itwill typically be used. The terms used herein are not intended to belimited to any particular narrow interpretation unless clearly statedotherwise, either expressly or impliedly, in this document.

Magnetic resonance imaging (MRI) is a noninvasive method of producinghigh quality images of the interior of the human body. It allows medicalpersonnel to see inside the human body without surgery or the use ofionizing radiation such as X-rays. The images are of such highresolution that disease and other forms of pathology can often bevisually distinguished from healthy human tissue. Magnetic resonancetechniques and systems have also been developed for performingspectroscopic analyses by which the chemical content of tissue or othermaterial can be ascertained.

MRI uses a powerful magnet, radio waves and computer technology tocreate detailed images of the soft tissues, muscles, nerves and bones inthe human body. It does so by taking advantage of a basic property ofthe hydrogen atom, an atom that is found in abundance in all cellswithin the human body. In the absence of a magnetic field, the nuclei ofhydrogen atoms spin like a top, or precess, randomly in every direction.When subject to a strong magnetic field, however, the spin-axes of thehydrogen nuclei align themselves in the direction of that field. This isbecause the nucleus of the hydrogen atom has what is referred to as alarge magnetic moment, which is basically a strong inherent tendency toline up with the direction of the magnetic field. Collectively, thehydrogen nuclei of the area to be imaged create an average vector ofmagnetization that points parallel to the magnetic field.

A typical MRI system, or scanner, includes a main magnet, three gradientcoils, a radio frequency (RF) antenna (often referred to as the wholebody coil), and a computer station from which an operator can controlthe overall MRI system. The chief component of the MRI system, however,is the main magnet. It is typically superconducting in nature andcylindrical in shape. Within its cylindrical bore (into which patientsare placed during an MRI procedure), the main magnet generates a strongmagnetic field, often referred to as the B₀ field, which is both uniformand static (non-varying). This B₀ magnetic field is oriented along thelongitudinal axis of the bore, referred to as the z direction, whichcompels the magnetization vectors of the hydrogen nuclei in the body toalign themselves in that direction. In this alignment, the nuclei areprepared to receive RF energy of the appropriate frequency from thewhole body coil. This frequency is known as the Larmor frequency and isgoverned by the equation ω=ΥB₀, where ω is the Larmor frequency (atwhich the hydrogen atoms precess), Υ is the gyromagnetic constant, andB₀ is the strength of the magnetic field.

The RF antenna, or whole body coil, is generally used both to transmitpulses of RF energy and to receive the resulting magnetic resonance (MR)signals induced thereby in the hydrogen nuclei. Specifically, during itstransmit cycle, the body coil broadcasts RF energy into the cylindricalbore. This RF energy creates a radio frequency magnetic field, alsoknown as the RF B₁ field, whose magnetic field lines are directed in aline perpendicular to the magnetization vector of the hydrogen nuclei.The RE pulse (or B1 field) causes the spin-axes of the hydrogen nucleito tilt with respect to the main (B₀) magnetic field, thus causing thenet magnetization vector to deviate from the z direction by a certainangle. The Rf pulse, however, will affect only those hydrogen nucleithat are precessing about their axes at the frequency of the RF pulse.In other words, only the nuclei that “resonate” at that frequency willbe affected, and such resonance is achieved in conjunction with theoperation of the three gradient coils.

The gradient coils are electromagnetic coils. Each gradient coil is usedto generate a linearly varying yet static magnetic field along one ofthe three spatial directions (x,y,z) within the cylindrical bore knownas the gradient B₁ field. Positioned inside the main magnet, thegradient coils are able to alter the main magnetic field on a very locallevel when they are turned on and off very rapidly in a specific manner.Thus, in conjunction with the main magnet, the gradient coils can beoperated according to various imaging techniques so that the hydrogennuclei—at any given point or in any given strip, slice or unit ofvolume—will be able to achieve resonance when an RF pulse of theappropriate frequency is applied. In response to the RF pulse, theprecessing hydrogen atoms in the selected region absorb the RF energybeing transmitted from the body coil, thus forcing the magnetizationvectors thereof to tilt away from the direction of the main (B₀)magnetic field. When the body coil is turned off, the hydrogen nucleibegin to release the RF energy in the form of the MR signal, asexplained further below.

One well known technique that can be used to obtain images is referredto as the spin echo imaging technique. Operating according to thistechnique, the MRI system first activates one gradient coil to set up amagnetic field gradient along the z-axis. This is called the “sliceselect gradient,” and it is set up when the RF pulse is applied and isshut off when the RF pulse is turned off. It allows resonance to occuronly within those hydrogen nuclei located within a slice of the areabeing imaged. No resonance will occur in any tissue located on eitherside of the plane of interest. Immediately after the RF pulse ceases,all of the nuclei in the activated slice are “in phase,” i.e., theirmagnetization vectors all point in the same direction. Left to their owndevices, the net magnetization vectors of all the hydrogen nuclei in theslice would relax, thus realigning with the z direction. Instead,however, the second gradient coil is briefly activated to create amagnetic field gradient along the y-axis. This is called the “phaseencoding gradient.” It causes the magnetization vectors of the nucleiwithin the slice to point, as one moves between the weakest andstrongest ends of the gradient, in increasingly different directions.Next, after the RF pulse, slice select gradient and phase encodinggradient have been turned off, the third gradient coil is brieflyactivated to create a gradient along the x-axis. This is called the“frequency encoding gradient” or “read out gradient,” as it is onlyapplied when the MR signal is ultimately measured. It causes therelaxing magnetization vectors to be differentially re-excited, so thatthe nuclei near the low end of the gradient begin to precess at a fasterrate, and those at the high end pick up even more speed. When thesenuclei relax again, the fastest ones (those which were at the high endof the gradient) will emit the highest frequency of radio waves.

Collectively, the gradient coils allow the MR signal to be spatiallyencoded, so that each portion of the area being imaged is uniquelydefined by the frequency and phase of its resonance signal. Inparticular, as the hydrogen nuclei relax, each becomes a miniature radiotransmitter, giving out a characteristic pulse that changes over time,depending on the local microenvironment in which it resides. Forexample, hydrogen nuclei in fats have a different microenvironment thando those in water, and thus transmit different pulses. Due to thesedifferences, in conjunction with the different water-to-fat ratios ofdifferent tissues, different tissues transmit radio signals of differentfrequencies. During its receive cycle, the body coil detects theseminiature radio transmissions, which are often collectively referred toas the MR signal. From the body coil, these unique resonance signals areconveyed to the receivers of the MR system where they are converted intomathematical data corresponding thereto. The entire procedure must berepeated multiple times to form an image with a good signal-to-noiseratio (SNR). Using multidimensional Fourier transformations, MR systemcan convert the mathematical data into a two- or even athree-dimensional image.

When more detailed images of a specific part of the body are needed, alocal coil is often used in addition to, or instead of, the whole bodycoil. A local coil can take the form of a volume coil or a surface coil.A volume coil is used to surround or enclose the volume to be imaged(e.g., a head, an arm, a wrist, a leg, a knee or other region ofinterest). A surface coil, however, is merely fitted or otherwise placedagainst a particular surface of the patient so that the underlyingregion of interest can be imaged (e.g., the abdominal, thoracic and/orpelvic regions). In addition, a local coil can be designed to operateeither as a receive-only coil or a transmit/receive (T/R) coil. Areceive-only coil is only capable of detecting the MR signals producedby the human body (in response to the B₁ magnetic field generated by theMR system during a scanning procedure). A T/R coil, however, is capableof both receiving the MR signals as well as transmitting the RF pulsesthat produce the RF B₁ magnetic field, which is the prerequisite forinducing resonance in the tissues of the region of interest.

It is well known in the field of MRI to use a single local coil, whethersurface or volume, to detect the MR signals. According to the singlecoil approach, a relatively large local coil is used to cover or enclosethe entire region of interest. Early receiving coils were just linearcoils, meaning that they could detect only one of the two (i.e.,vertical M_(X′) and horizontal M_(Y′)) quadrature components of the MRsignals produced by the region of interest. Subsequent receiving coils,however, employed quadrature mode detection, meaning that they couldintercept both the vertical and horizontal components. Compared tolinear receiving coils, quadrature receiving coils enabled MRI systemsto provide images for which the SNR was much improved, typically by asmuch as 41%. Even with the improvement brought with quadrature modedetection, the single coil approach still provided images whose qualityinvited improvement. The disadvantage inherent to the single coilapproach is attributable to just one coil structure being used toacquire the MR signals over the entire region of interest.

Phased array coils were developed to overcome the shortcomings with thesingle coil approach. Instead of one large local coil, the phased arrayapproach uses a plurality of smaller local coils, with each such coilcovering or enclosing only a portion of the region of interest. In asystem having two such coils, for example, each of the coils would coveror enclose approximately half of the region of interest, with the twocoils typically being partially overlapped for purposes of magneticisolation. The two coils would acquire the MR signals from theirrespective portions simultaneously, and they would not interactadversely due to the overlap. Because each coil covers only half of theregion of interest, each such coil is able to receive the MR signals ata higher SNR ratio for that portion of the region of the interest withinits coverage area. The smaller local coils of the phased array thuscollectively provide the MRI system with the signal data necessary togenerate an image of the entire region of interest that is higher inresolution than what can be obtained from a single large local coil.

One example of a phased array coil is the Gore® torso array produced byW. L. Gore and Associates, Inc. The torso array contains four surfacecoils, two of which disposed in an anterior paddle and the other twodisposed in a posterior paddle. The two paddles are designed to beplaced against the anterior and posterior surfaces, respectively, of thepatient about the abdominal, thoracic and pelvic regions. The torsoarray is designed for use with an MR system whose data acquisitionsystem has multiple receivers. The four leads of the torso array, oneeach from the two anterior surface coils and the two posterior surfacecoils, can be connected to separate receivers, with each receiveramplifying, and digitizing the signal it receives. The MR system thencombines the digitized data from the separate receivers to form an imagewhose overall SNR is better than what could be obtained from a singlelocal coil, or even two larger anterior and posterior local coils,covering the entire region of interest alone.

It is also well known to obtain images of internal bodily structuresthrough the use of intracavity probes. An example of a prior artintracavity probe can be found in U.S. Pat. Nos. 5,476,095 and5,355,087, both of which are assigned to the assignee of the presentinvention and incorporated herein by reference. The prior art probedisclosed in those patents is designed to be inserted into bodilyopenings such as the rectum, vagina, and mouth. Those patents alsodisclose interface devices that are designed to interface the prior artintracavity probe with MR imaging and spectroscopy systems. A method ofusing the intracavity probe is disclosed in U.S. Pat. No. 5,348,010,which is also assigned to the assignee of the present invention andincorporated herein by reference.

The prior art probe, operated in conjunction with its associatedinterface unit, allows an MR system to generate images of, andspectroscopic results for, various internal bodily structures such asthe prostate gland, colon or cervix. Examples of such prior art probesinclude the BPX-15 prostate/endorectal coil (E-coil), the PCC-15colorectal coil, and the BCR-15 cervix coil, all of which are part ofthe MRInnervu® line of disposable coils produced by MEDRAD, Inc. ofIndianola, Pa. Examples of such interface units include the ATD-II andthe ATD-Torso units, also produced by MEDRAD, Inc.

The ATD-II unit is used to interface the prior art probe with onereceiver of an MR system to provide images or spectra of the region ofinterest, namely, the prostate gland, colon or cervix. The ATD-Torsounit is used to interface not only the prior art probe but also theGore® torso array with multiple receivers of the MR system. Whenconnected to such a probe and the torso array, the ATD-Torso unit allowsthe MR system to provide images or spectra not only of the prostategland, colon or cervix but also of the surrounding anatomy, i.e., theabdominal, thoracic and pelvic regions.

Despite their widespread acceptance and good reputation in themarketplace, these prior art intracavity probes and interfaces unitsnevertheless have a few shortcomings. First, the prior art probe and itsassociated interface units (i.e., ATD-II and ATD Torso units) aredesigned to operate only with 1.0 or 1.5 Tesla MR systems. Consequently,they are not suitable for use with MR systems designed to operate athigher field strengths, such as the 2.0 to 5.0 Tesla and particularly3.0 Tesla MR systems that are capable of producing even higher qualityimages and spectrographic results. Second, as a result of that designconstraint, the prior art intracavity probe was designed with a coilloop that exhibits a 750 to 1000 ohm output impedance. Consequently, theinterface units for the prior art probe had to include a π network orsimilar circuitry to match the high output impedance of the coil loop tothe low input impedance (e.g., 50 ohms) required by various MR systems.Third, the design of the prior art probe allowed the tuning of its coilloop to deviate from the operating frequency of the MR system, theextent to which depending on the particular conditions (e.g., patients)in which the probe was used. Therefore, the prior art interface unitsfor the prior art probe typically had to include tuning circuitry so asto assure that the intracavity probe could be tuned to the operatingfrequency of the MR system under all operating conditions.

SUMMARY OF THE INVENTION

Several objectives of the invention and other advantages are attained bythe various embodiments and related aspects of the invention summarizedbelow.

In one aspect of a presently preferred embodiment, the inventionprovides an intracavity probe for use with a magnetic resonance (MR)system for obtaining images or spectra of a region of interest within acavity of a patient. The intracavity probe includes a first coil loopand first and second output cables. Designed to receive MR signals fromthe region of interest, the first coil loop has a plurality ofcapacitors including first and second drive capacitors and a tuningcapacitor. The first and second drive capacitors are serially connectedwithin the first coil loop and at a junction node thereof form a virtualground for electrically balancing and impedance matching the first coilloop. The first and second drive capacitors are of approximately equalvalue. The tuning capacitor is serially connected within the first coilloop diametrically opposite the junction node of the two drivecapacitors. The tuning capacitor has a value selected to resonate thefirst coil loop at an operating frequency of the MR system. The firstoutput cable is connected at one end thereof across the first drivecapacitor, and the second output cable is connected at one end thereofacross the second drive capacitor. The first and second output cableseach have an electrical length of S_(L)+n(λ/4) wherein S_(L) is asupplemental length whose reactance is of a same magnitude as areactance of the drive capacitor corresponding thereto, n is an oddinteger, and λ is a wavelength of the operating frequency. The twooutput cables terminate in a plug therefor for connecting the first coilloop to an interface device for the intracavity probe.

In another aspect of the presently preferred embodiment, the inventionprovides an interface device for interfacing an intracavity probe with a(probe) input port of a magnetic resonance (MR) system which is notequipped with its own preamplifier. The MR system has a receive cycleand a transmit cycle of operation, and the intracavity probe has a coilloop and a pair of output cables for connecting the coil loop to theinterface device. The interface device includes a phase shifting networkand a preamplifier. During the receive cycle, the phase shifting networkenables the coil loop to be coupled through the output cables to theprobe input port of the MR system and permits MR signals received fromeach of the output cables to be constructively combined. During thetransmit cycle, the phase shifting network enables the coil loop to bedecoupled through the output cables from a transmit field of the MRsystem. The preamplifier provides gain and impedance matching betweenthe phase shifting network and the probe input port so that the MRsignals received from the phase shifting network are passed withenhancement of signal-to-noise ratio to the probe input port during thereceive cycle.

In a related aspect of the presently preferred embodiment, the inventionprovides an interface device for interfacing an intracavity probe with a(probe) input port of a magnetic resonance (MR) system which is equippedwith its own preamplifier. The MR system has a receive cycle and atransmit cycle of operation, and the intracavity probe has a coil loopand a pair of output cables for connecting the coil loop to theinterface device. The interface device includes a phase shiftingnetwork. During the receive cycle, the phase shifting network enablesthe coil loop to be coupled through the output cables to the probe inputport of the MR system and permits MR signals received from each of theoutput cables to be constructively combined and routed to the probeinput port. During the transmit cycle, the phase shifting networkenables the coil loop to be decoupled through the output cables from atransmit field of the MR system.

In yet another aspect of the presently preferred embodiment, theinvention provides an interface device for interfacing both anintracavity probe and a coil system with a magnetic resonance (MR)system. The MR system has a receive cycle and a transmit cycle ofoperation, and the intracavity probe has a coil loop and a pair ofoutput cables for connecting the coil loop to the interface device. Theinterface device includes a phase shifting network and an arrayinterface circuit. During the receive cycle, the phase shifting networkenables the coil loop to be coupled through the output cables to a probeinput port of the MR system and permits MR signals received from each ofthe output cables to be constructively combined and routed to the probeinput port. During the transmit cycle, the phase shifting networkenables the coil loop to be decoupled through the output cables from atransmit field of the MR system. The array interface circuit is forelectrically interconnecting one or more coil elements of the coilsystem and other input ports of the MR system.

In a variation of the presently preferred embodiment, the inventionprovides an intracavity probe for use with a magnetic resonance (MR)system for obtaining images or spectra of a region of interest within acavity of a patient. The intracavity probe includes a first coil loopand an output cable. Designed to receive MR signals from the region ofinterest, the first coil loop has a plurality of capacitors seriallyconnected therein. The plurality of capacitors includes a drivecapacitor and a tuning capacitor. The drive capacitor is used forelectrically balancing and impedance matching the first coil loop. Thetuning capacitor is positioned diametrically opposite the drivecapacitor and has a value selected to resonate the first coil loop at anoperating frequency of the MR system. An output cable is connected atone end thereof across the drive capacitor. The output cable has anelectrical length of S_(L)+n(λ/4) wherein S_(L) is a supplemental lengthwhose reactance is of a same magnitude as a reactance of the drivecapacitor, n is an odd integer, and λ is a wavelength of the operatingfrequency. The output cable terminates in a plug therefor for connectingthe first coil loop to an interface device for the intracavity probe.

It should be understood that the present invention is not limited to thepresently preferred embodiment(s) and related aspects discussed above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its presently preferred and alternative embodimentswill be better understood by reference to the detailed disclosure belowand to the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of a coil loop and an output cable of anintracavity probe according to one aspect of a first embodiment of theinvention;

FIG. 2 is a perspective view showing the intracavity probe of FIG. 1 inits fully assembled and fully equipped state;

FIG. 3 is a cross-sectional view of the intracavity probe taken throughline 3-3 of FIG. 2 showing a distal end of the probe and inflatableballoon(s) attached thereto;

FIG. 4 is partial cross-sectional view of the intracavity probe takenthrough line 4-4 of FIG. 2 showing its shaft in cross-section and thetwo lumens defined therein and an anti-migration disc snapped onto theshaft;

FIG. 5 is a cross-sectional view of the distal end of the intracavityprobe taken through line 5-5 of FIG. 3 showing its outer and innerballoons, its coil loop situated between the balloons, and its shaftwith the two lumens defined therein;

FIG. 6 is a cross-sectional view of the distal end of the intracavityprobe taken through line 6-6 of FIG. 3 showing its coil loop situatedatop an anterior surface of the inner balloon;

FIG. 7 is a cross-sectional view of the shaft of the intracavity probeof FIG. 2 illustrating the two lumens defined therein and a flexible tipat its distal end;

FIG. 8 is a schematic diagram of an interface device according toanother aspect of the first embodiment of the invention wherein, in itssingle-receiver version, the interface device has a probe interfacecircuit for interfacing the intracavity probe of FIGS. 1-7 with a(probe) input port of a magnetic resonance (MR) system which is notequipped with its own preamplifier;

FIG. 9 is a schematic diagram of an interface device according to yetanother aspect of the first embodiment of the invention wherein, in itsmultiple-receiver version, the interface device has (i) a probeinterface circuit for interfacing the intracavity probe of FIGS. 1-7with a (probe) input port of an MR system equipped with its ownpreamplifier and (ii) an array interface circuit for interfacing aphased array coil system, such as the Gore® torso array, with the (coil)input ports of the MR system;

FIG. 10 is a perspective view of the interface device in itssingle-receiver version of FIG. 8, which is designed to interface theintracavity probe to the MR system via a (probe) input port thereof thatis not equipped with a preamplifier;

FIG. 11 is a perspective view of the interface device in itsmultiple-receiver version of FIG. 9, which is designed to interface theintracavity probe and a phased array coil system, such as the Gore®torso array, with the Phased Array Port of the MR system;

FIG. 12 is a schematic diagram of a coil loop and an output cable of anintracavity probe, and a decoupling diode of an interface devicecorresponding thereto, according to a first alternative embodiment ofthe invention;

FIG. 13 is a schematic diagram of a coil loop and an output cable of anintracavity probe, and decoupling diodes of an interface devicecorresponding thereto, according to a second alternative embodiment ofthe invention;

FIG. 14 is a schematic diagram of a coil loop and an output cable of anintracavity probe, and a decoupling diode of an interface devicecorresponding thereto, according to a third alternative embodiment ofthe invention;

FIG. 15 is a partially exploded schematic view of a coil loop and theoutput cables therefor according to a presently preferred embodiment ofthe invention;

FIG. 16 is a schematic diagram of an interface device according toanother aspect of the presently preferred embodiment of the inventionwherein, in its single-receiver version, the interface device has aprobe interface circuit for interfacing the intracavity probe of FIG. 15with a (probe) input port of a magnetic resonance (MR) system which isnot equipped with its own preamplifier;

FIG. 17 is a perspective view of the interface device in itssingle-receiver version of FIG. 16, which is designed to interface theintracavity probe to the MR system via a (probe) input port thereof thatis not equipped with a preamplifier;

FIG. 18 is a schematic diagram of an interface device according to yetanother aspect of the presently preferred embodiment of the inventionwherein, in its multiple-receiver version, the interface device has (i)a probe interface circuit for interfacing the intracavity probe of FIG.15 with a (probe) input port of an MR system equipped with its ownpreamplifier and (ii) an array interface circuit for interfacing a coilsystem, such as the Gore® torso array, with the (coil) input ports ofthe MR system;

FIG. 19 is a perspective view of the interface device in itsmultiple-receiver version of FIG. 18, which is designed to interface theintracavity probe and a coil system, such as the Gore® torso array, withthe Phased Array Port of the MR system; and

FIGS. 20A and 20B illustrate top and side schematic views of anintracavity probe having two or more coil loops deployed in a phasedarray configuration.

DETAILED DESCRIPTION OF THE INVENTION

In all of its embodiments and related aspects, the present inventiondisclosed below is ideally used with magnetic resonance (MR) systemsdesigned to operate at 3.0 Tesla field strengths; though it is alsoapplicable to those operable from approximately 2.0 to 5.0 T. Forpurposes of illustration below, the invention will be described in thecontext of the 3.0T systems produced by General Electric Medical Systems(GEMS).

FIGS. 1-7 illustrate one aspect of a first embodiment of the invention,namely, an intracavity probe, generally designated 1. The probe isintended for use with an MR system to obtain images or spectra of aregion of interest within a cavity of a patient. It is described hereinin a specific implementation, i.e., as an endorectal probe designed tobe inserted into the rectum to obtain images and/or spectra of the maleprostate gland. Although presented herein as an endorectal probe, itshould be understood that the invention is equally capable of beingadapted to obtain images of and/or spectra from other regions ofinterest such as those accessible through the mouth, the vagina or otherorifices penetrable by an intracavity probe. The principles presentedherein may also be applied to MR imaging or spectroscopic techniquesappropriate for the arteries, veins, and other structures of the body.Whatever the application, the receiving coil within intracavity probewill need to be housed in, or otherwise incorporated into, a packageappropriately designed to conform to the target anatomy.

In its most novel aspects as best shown in FIG. 1, intracavity probe 1includes a coil loop 2 and an output cable 3. Ideally made of aconductive material that is flexible, coil loop 2 is preferably a singleturn coil capable of picking up radio frequency (RF) signals. Designedto receive magnetic resonance RF signals from the region of interest,coil loop 2 has a plurality of capacitors including first drivecapacitor 21, second drive capacitor 23, and tuning capacitor 24. Thefirst and second drive capacitors are serially connected within coilloop 2. As is explained below, the junction node 22 at which drivecapacitors 21 and 23 connect forms a virtual ground for electricallybalancing and impedance matching the coil loop 2. Tuning capacitor 24 isalso serially connected within coil loop 2 but diametrically oppositethe junction node 22 of capacitors 21 and 23. Tuning capacitor 24 isselected to resonate coil loop 2 at an operating frequency of the MRsystem, which for a 3.0 Tesla scanner would be approximately 128 MHz.

Output cable 3 is designed to connect coil loop 2 to an interface devicefor the intracavity probe 1. Such an interface device, such as either ofthe ones disclosed below, at its other end in turn connects to a probeinput port of the MR system 10, as shown in FIGS. 8 and 9. Encasedwithin an insulating sheath, output cable 3 has a shield conductor 31and a center conductor 32 insulatively disposed therein. The shieldconductor 31 connects to the junction node 22, and the center conductor32 connects to a node of one of the drive capacitors 21 and 23 oppositejunction node 22, as shown in FIG. 1. In addition, for reasons detailedbelow, output cable 3 preferably has an electrical length ofn(λ/2)+S_(L), where n is an integer, λ is the wavelength of theoperating frequency of MR system 10, and S_(L) is a supplemental length.

FIG. 2 shows the intracavity probe 1 of the present invention in fullyassembled form, and FIGS. 3-7 illustrate various partial cross-sectionalviews thereof. Intracavity probe 1 includes a flexible shaft 40 andinner and outer balloons 50 and 60. The shaft 40 has a distal end whosetip 41 is preferably substantially more flexible than the remainder ofthe shaft, and indeed may be bonded thereto as indicated at referencenumeral 15. The use of such a flexible tip 41 will reduce not only thediscomfort felt by the patient but also the likelihood of perforatingnearby tissue during use of the probe.

Inner balloon 50 connects to the distal end of shaft 40 and encloses tip41 thereof, as best shown in FIG. 3. Inner balloon 50 is generallycylindrical in shape except for a substantially planar section on itsanterior surface 51. It can be anchored to shaft 40 by a clamp 16 and byan interference fit with the distal end of shaft 40. Coil loop 2 itselfis preferably encased within 5K volt insulation over which shrink wrapor similar tubing is used, thus providing a double layer of insulation.A non-stretchable material 55, which is preferably composed of anadhesive-backed cloth, can then be used to attach the coil loop 2 to theanterior surface 51 of inner balloon 50, thus securing the coil loop 2between the inner and outer balloons 50 and 60.

Outer balloon 60 also connects to the distal end of shaft 40, enclosingboth coil loop 2 and inner balloon 50. It can be anchored to shaft 40 bya clamp 17 and by an interference fit with the distal end. Outer balloon60 has anterior and posterior surfaces 61 and 62. The anterior surface61 is preferably saddle-shaped to conformably fit against acorrespondingly-shaped interior surface/contour of the cavity, which inthe case of the prostate probe will be the rectal prostatic bulgeinferior to the ampulla of the rectum. The posterior surface 62 featuresat least one pair of undulating folds 63 projecting therefrom. Asdescribed below, these folds 63 enable the outer balloon 60 to properlyposition the coil loop 2 in operative proximity to the rectal prostaticbulge of the patient when the inner balloon 50 is inflated, whichoptimizes the coupling between coil loop 2 and the target anatomy. Inaddition, as shown in FIG. 5, lateral indentations 64 are preferablyprovided within outer balloon 60 intermediate the anterior and posteriorsurfaces 61 and 62. These indentations 64 essentially form a shelf onwhich the sides of coil loop 2 rest during assembly of the probe 1. Theyessentially serve as a means of positioning the coil loop between thosesurfaces when balloons 50 and 60 are in the uninflated state. Theballoons 50 and 60 are each preferably made of a medical-grade latex orother appropriate elastomeric material. Such material should, of course,be non-paramagnetic and exhibit low dielectric losses.

Flexible shaft 40 defines two lumens 42 and 44, as best shown in FIGS.3, 4, 5 and 7. Within its cylindrical wall near its distal end, shaft 40also defines a hole 43 in communication with lumen 42, as shown in FIG.7. Lumen 42 and hole 43 together serve as a passageway for the fluid(e.g., gas or liquid) pumped into and expelled out of inner balloon 50when inflated and deflated, respectively. Further away from its distalend, shaft 40 defines another hole 45 in its cylindrical wall. Lumen 44and hole 45 act as the conduit through which output cable 3 is routedfrom coil loop 2. Output cable 3, as shown in FIG. 2, has a plug 35 atits proximal end to connect the intracavity probe 1 with the appropriateinterface device.

Intracavity probe 1 further includes an anti-migration disc 46, anintroducer 48, and a handle 49. Fixed to the proximal end of shaft 40,the handle 49 enables the probe 1 to be easily manipulated as its distalend, inclusive of outer balloon 60 secured thereon, is inserted into therectum and appropriately aligned within the cavity as described below.The introducer 48, also referred to as a dilator element, is designed tobe easily slid over the entire length of shaft 40. Preferablyfunnel-shaped, the introducer 48 can be used to manually dilate the analsphincter to allow outer balloon 60 to be easily positioned within thecavity. Without introducer 48, the anal sphincter would contract aroundshaft 40 and interfere with the ability to rotationally andlongitudinally position the intracavity probe 1 within the cavity. Theanti-migration disc 46, composed of a semi-rigid plastic or othersuitable polymer, is preferably semi-spherical in shape. As shown inFIGS. 2 and 4, the disc 46 defines a slot 47. This slot allows the disc46 to be snapped onto shaft 40. When affixed to shaft 40 adjacent theanal sphincter after the probe has been inserted into the rectum, theanti-migration disc 46 prevents the probe 1 from migrating superiorlydue to the normal peristaltic activity of the colon.

Intracavity probe 1 also includes a means for controlling inflation ofinner balloon 50. The inflation control means preferably takes the formof a compressible inflator cuff 70, a tube 71, and a stop cock 72. Asyringe of suitable size could be used in lieu of cuff 70. Tube 71connects the inflator cuff 70, or syringe, to the lumen 42 at theproximal end of shaft 40. The stop cock 72 is connected in series withtube 71 and serves to control whether air is pumped to or released frominner balloon 50. The probe 1 also preferably features a scale 14printed on an outer surface of shaft 40. Scale 14 provides an indicationof not only the distance that shaft 40 has been inserted into the cavitybut also the rotational orientation of the distal end for properalignment of the saddle-shaped anterior surface 61 of outer balloon 60with the prostate.

In operation, the distal end of intracavity probe 1 is inserted into thecavity via the rectum while inner balloon 50, and outer balloon 60surrounding it, are in the uninflated state. Once the distal end isinserted, the introducer 48 can be used to keep the anal sphincterdilated and thereby enable shaft 40, and its balloon-enclosed distalend, to be easily manipulated within the cavity. With the distal endinserted and the introducer 48 in place, the scale 14 on shaft 40 canthen serve as a guide to enable the clinician or other medical personnelto more accurately position the probe both rotationally andlongitudinally within the cavity adjacent the region of interest. Oncethe intracavity probe 1 is correctly positioned, the introducer 48 canbe pulled inferiorly along the shaft, thereby allowing the sphincter tocontract around shaft 40. This contraction assists in holding theintracavity probe 1 in place. The anti-migration disc 46 can then besnapped onto the shaft 40 adjacent the sphincter to assure that theintracavity probe 1 stays in position during the MR scanning procedure.

Before inflating the balloons, the stop cock 72 must be switched to theopen state. By pumping inflator cuff 70, the inner balloon 50 willinflate via tube 71, stop cock 72, and lumen 42 and hole 43 in shaft 40.As the inner balloon inflates, the non-stretchable material 55 thatsecures coil loop 2 to the anterior surface 51 of inner balloon 50 alsofocuses inflation of the inner balloon posteriorly so as to inflate intothe undulating folds 63 of outer balloon 60. As the undulating folds 63inflate, they soon force the posterior surface 62 (i.e., folds 63) ofouter balloon 60 to abut against a wall of the cavity opposite theregion of interest. As inner balloon 50 continues to inflate, the forceof inflation is then directed towards the underside of the anteriorsurface 61 of outer balloon 60. The anterior surface 51 of inner balloon50, with coil loop 2 attached thereto, thus forces the saddle-shapedanterior surface 61 of outer balloon 60 against thecorrespondingly-shaped interior contour of the cavity, namely, theprostatic region of the rectum. Once the balloons at the distal end arefully inflated, the coil loop 2 will be situated approximate theprostate gland for optimal reception of the MR signals therefrom duringthe MR scanning procedure. The stop cock 72 can then be switched to theclosed position, thereby allowing the clinician to disconnect theinflator cuff 70 without deflating the balloons 50 and 60. Theintracavity probe 1 can then be connected to the appropriate interfacedevice via the plug 35 of output cable 3.

When the scanning procedure is completed, the clinician need only switchthe stop cock 72 to the open position to deflate inner balloon 50 andouter balloon 60 therewith. Whether or not the anti-migration disc 46 isremoved from shaft 40, the balloon-enclosed distal end can then beremoved from the rectum merely by gently pulling on the handle 49 of theintracavity probe 1.

Alternatively, the intracavity probe 1 may employ a single balloon inlieu of the double balloon version described above. It may be composedof a single-ply medical-grade latex material or other suitableelastomeric material. In this arrangement, the balloon still connects tothe distal end of flexible shaft 40, and the balloon will preferablyhave anterior and posterior surfaces identical to those described forthe double balloon version. The coil loop 2, however, will ideally bebonded or otherwise secured to the underside of the anterior surface 61of the balloon. The coil loop 2 could also be encapsulated within theanterior surface 61 during the process of manufacturing the balloon. Forexample, coil loop 2 could be placed on a surface of the balloon andthen the balloon could be redipped to place another ply of material overthe outer surface of the balloon, thus covering coil loop 2 and creatingthe anterior surface 61 described above. However manufactured, when theinflatable balloon is inserted into the cavity and inflated, theundulating folds 63 will press against the wall of the cavity oppositethe region of interest. Upon full inflation of the balloon, its anteriorsurface 61 will then be forced against the correspondingly-shapedinterior contour of the cavity thereby bringing the coil loop 2 intooperative proximity with the region of interest (i.e., the prostategland) wherefrom it can best receive the MR signals.

The invention further provides a method of designing the intracavityprobe 1. Variations on this method, which will become apparent toskilled artisans upon reading this document, are also contemplated bythe present invention. The first step of the method involves choosingthe size of a loop of wire that will form the basis for coil loop 2. Foran intracavity probe designed for imaging the prostate, the wire loopshould be sized so that the distal end of the probe, inclusive of thetwo balloons between which coil loop 2 will be situated, can be insertedinto the rectum with minimal discomfort to the patient. The next stepsinvolve temporarily inserting a variable capacitor within the wire loop,and then subjecting the loop to the operating frequency of the MR system10. For 3.0 Tesla scanners for which the present invention isparticularly well suited, the operating frequency would be approximately128 MHz. For the GEMS 3.0T Signa® scanner, the operating frequency isactually closer to 127.74 MHz. For the Siemens 3.0T scanner, theoperating frequency is 123.2 MHz.

While the wire loop is being subjected to RF energy at the designatedoperating frequency, the variable capacitor should be adjusted to avalue, hereinafter referred to as C_(RV), at which the wire loop willresonate. Once resonance is achieved, the capacitive and inductivereactances of the wire loop will, of course, be equal in magnitude atthe operating frequency. For the purposes of the following calculations,10 picofarads (pF) is an ideal value for C_(RV) to establish resonancewithin the wire loop according to this method of designing theintracavity probe 1.

Once C_(RV) has been established, the quality factor of the loop can bemeasured while the loop is operating under loaded conditions. There areseveral known techniques for measuring the quality factor. One suchtechnique involves: making an S₂₁ response measurement using two testprobes and a network analyzer, with the two test probes being connectedto ports 1 and 2, respectively, of the network analyzer. With the loopsof the two test probes positioned at right angles to each other, thewire loop of the present invention is placed between them. Thisarrangement allows RF energy supplied to the loop of the first testprobe to be induced within the wire loop, which in turn induces an RFsignal in the loop of the second test probe. The two test probes thenconvey their respective RF signals to the network analyzer, whichdisplays the resulting frequency response curve graphically in terms ofamplitude versus frequency. Using the displayed signal, the qualityfactor can be ascertained by locating the center frequency of thefrequency response curve and dividing it by the 3 dB bandwidth (i.e.,the band between the 3 dB (half power) points at the high-pass andlow-pass ends of the curve). For a 3.0 Tesla scanner, the quality factorof the loop will lie between 10 and 20. More typically, the qualityfactor of the loop under loaded conditions will be:Q_(Loaded)=15(measured)

The next step of the method involves determining the series resistance,R_(S), of the loop. The series resistance represents the equivalentresistive losses exhibited by the loop due to its presence within thecavity of the patient. R_(S) is thus not a physical component, only theeffect the patient has on the loop. It reduces the quality of coil loop2 by partially dissipating the energy within it. It can be calculatedfrom the equation:R _(S) =X _(L) /Qwhere Q is the quality factor measured above and X_(L) is the inductivereactance of the wire loop when loaded. As noted above, the capacitiveand inductive reactances of the loop are equal in magnitude atresonance:X_(L)=X_(P)X _(L)=2πfL _(COIL) and X _(P)=1/(2πfC _(RV))where f is the operating frequency of the MR system 10. Consequently,the inductive reactance of the loop, X_(L), can be calculated from:X _(L)=1/(2πfC _(RV))=1/(2π×128×10⁶×10×10⁻¹²)=124.34Ω.

Consequently, the series resistance of the loop will be:R _(S) =X _(L) /Q _(Loaded)=124.34Ω/15=8.29Ω.

The method also requires the step of matching the output impedance ofintracavity probe 1 with the impedance required by the external circuitwith which the intracavity probe shall interface. The external circuitcan take the form of one of the interface devices disclosed herein, andwill typically require an impedance of 50Ω. Consequently, this step ofthe method includes devising an impedance matching network to match theimpedance required by the external circuit, R_(P), to the seriesresistance of the loop, R_(S). In this impedance matching network, thequality of the series and parallel legs of the matching network, asrepresented by Q_(P)=R_(P)/X_(P) and Q_(S)=X_(S)/R_(S), are equal.Consequently, R_(S) and R_(P) are related by the equation:R _(P)=(Q ²+1)R _(S)where R_(P) can also be referred to as the equivalent parallelresistance. Given that the quality of the series and parallel legs ofthe matching network are equal, the quality of the matching network canthen be calculated from:Q=Q _(S,P)=(R _(P) /R _(S)−1)^(1/2)=(50Ω/8.29Ω−1)^(1/2)=2.24.

The parallel reactance, X_(P), associated with R_(P) in the impedancematching network can then be calculated from:X _(P) =R _(P) /Q=50Ω/2.24=22.32Ω.

The value of the matching capacitor can then determined from theparallel reactance:C _(P)=1/(2πfX _(P))=1/(2π×128×10⁶×22.32)=55.7 pF.

Another step involves inserting two capacitors of the matching valueinto the wire loop in series with each other. These are the two drivecapacitors, C_(D1) and C_(D2), respectively designated 21 and 23, shownin FIG. 1. Using the above calculations, the drive capacitors 21 and 23together have an effective value of 27.85 pF. Junction node 22 is formedat the site at which drive capacitors 21 and 23 connect. The shieldconductor 31 of output cable 3 connects to junction node 22, and thecenter conductor 32 connects to the node on the other side of eitherdrive capacitor 21 or drive capacitor 23. Therefore, according to theabove calculations, the value of drive capacitor 21, C_(D1), is thuswhat enables the coil loop 2 to appear as a 50 ohm source to theinterface device or other external circuit. This allows a 50 ohm coaxialcable to be used as the output cable 3.

A further step involves selecting a tuning capacitor, C_(TUN), so thatthe total capacitance of the wire loop is equal to the resonance value,C_(RV). The total capacitance of the wire loop, C_(RV), can bedetermined from:1/C _(RV)=1/C _(TUN)+1/C _(D1)+1/C _(D2)=1/C _(TUN)+2/C _(D)where C_(D)=C_(D1)=C_(D2). The value of the tuning capacitor, C_(TUN),can then be calculated as follows:

$\begin{matrix}{C_{TUN} = {\left( {C_{RV}*C_{D}} \right)/\left( {C_{D} - {2C_{RV}}} \right)}} \\{= {\left( {10 \times 10^{- 12}F \times 55.7 \times 10^{- 12}F} \right)/}} \\{\left( {{55.7 \times 10^{- 12}F} - {2 \times 10 \times 10^{- 12}F}} \right)} \\{= {15.6\mspace{14mu}{pF}}}\end{matrix}$

The variable capacitor is then removed from the wire loop and replacedwith the tuning capacitor, C_(TUN). Designated as C_(T) in FIG. 1, thetuning capacitor 24 is serially connected within the wire loopdiametrically opposite the junction node 22. Junction node 22 thus formsa virtual ground for electrically balancing the coil loop, as theelectric field there is effectively zero and the voltage drop acrosseach drive capacitor is equal but opposite in sign. This configurationresults in symmetry of the electric fields relative to the patientduring the receive cycle of MR system 10. It renders coil loop 2particularly sensitive to the magnetic field, but not the electricfield, components of the MR signals emitted by the region of interest.It thus enables coil loop 2 to receive the MR signals with a greatersignal-to-noise ratio than prior art probes. It also does so withgreater safety, as the voltages induced in the coil loop will be equaland half of what they would otherwise be if the coil loop were totallyunbalanced.

Due to the high operating frequency (e.g., 128 MHz for a 3.0T MR system)and very low operating Q (i.e., between 10-20) of coil loop 2, there isno need to tune coil loop 2 on a per patient or per-coil basis, unlikethe probe disclosed in U.S. Pat. Nos. 5,476,095 and 5,355,087. On thebasis of the above calculations including the quality factor of theloaded coil loop, the bandwidth of coil loop 2 will nominally be +/−4.25MHz. Consequently, assuming the coil loop will be built with +/−2%components, the tuning shift from probe to probe should be a maximum ofapproximately +/−1.85 MHz, which is substantially less than the 3 dBbandwidth of the coil loop even without the effects of the low inputimpedance preamplifier explained below. The tuning is essentially fixedwith out material compromise due to the low Q of coil loop 2 underloaded conditions.

In this embodiment, output cable 3 preferably has an electrical lengthof n(λ/2)+S_(L), where n is an integer, λ is the wavelength of theoperating frequency of MR system 10, and S_(L) is a supplemental length.As best shown in FIG. 1, the entire length of output cable 3 extendsfrom coil loop 2 to its plug 35. Plug 35 represents the point at whichthe output cable connects to the PIN diode 33, also referred to as thedecoupling diode, of the interface device or other external circuit. Then(λ/2) part yields a section whose length is one-half the operatingwavelength, which will effectively appear as zero electrical length. Thevalue of n will typically need only be equal to 1, as coil loop 2 willin practice always be reasonably close to the circuit to which it willconnect. S_(L) represents an additional section of output cable 3 whoseinductive reactance is ideally equal in magnitude to the capacitivereactance of first capacitor 21 across which the terminals of cable 3connect. The net effect is that the entire length of output cable 3exhibits an inductive reactance equal to the capacitive reactance offirst capacitor 21.

Supplement length S_(L) thus inherently acts as an inductor, hereinafterreferred to as L_(D), which affects the operation of intracavity probe1. During the transmit cycle of MR system 10, the MR system willdecouple coil loop 2 of intracavity probe 1 from the MR system byforward biasing PIN diode 33 with a 200 mA current (see, e.g., FIG. 8).This will effectively short circuit PIN diode 33, and leave the inherentinductor, L_(D), of output cable 3 and first drive capacitor 21, C_(D1),as a parallel resonant circuit. The high impedance of this parallelresonant circuit approximates an open circuit, which effectively openscoil loop 2 and thus decouples the intracavity probe 1 from the probeinput port of the host MR system 10. Conversely, during the receivecycle, the MR system will couple the intracavity probe 1 to the MRsystem by reverse biasing decoupling diode 33 with −5V DC. This willeffectively cause output cable 3 to act as a 50 ohm transmission linerather than an inductor, L_(D). This will allow coil loop 2 to detectthe MR signals generated within the region of interest by theresonance-inducing RF pulse transmitted by the body coil of MR system 10(or other external coil). The MR signals will then be passed to theinterface device via the conductors of cable 3.

Drive capacitors C_(D1) and C_(D2) will typically have values in therange of approximately 62 pF to 82 pF. Similarly, tuning capacitor 24,C_(TUN), will preferably be in the range of approximately 12 to 15 pF.Better decoupling (higher open-circuit impedance) during the transmitcycle can be obtained using a value for C_(D1) in the lower end of thepreferred range. Such a lower value for drive capacitor 21 would thenalso increase the source impedance that coil loop 2 presents to theinterface device during the receive cycle. Furthermore, the exact lengthof S_(L) will depend on the particular coil loop used within intracavityprobe 1. For a coil loop that would be only lightly loaded during use,for example, drive capacitors of, say, 120 pF may be used, in which caseS_(L) would be shorter. Conversely, for a coil loop that would be moreheavily loaded, drive capacitors of 40 pF might be used, in which caseS_(L) would be longer.

The intracavity probe 1 described above is particularly well suited foruse as an endorectal coil probe with the 3.0T MR systems produced byGEMS, although it should be understood that the probe can be used forother applications as well.

FIGS. 8 and 9 depict two other aspects of the first embodiment of theinvention, both of which designed to interface intracavity probe 1 withthe GEMS MR system. In its first aspect, the interface device interfacesthe intracavity probe with one receiver of the MR system, and is thusreferred to as the single-receiver version. In its second aspect, theinterface device interfaces both intracavity probe 1 and an externalcoil to the MR system using multiple receivers, and is referred to asthe multiple-receiver version. As is well known, the typical GEMS Signa®system features four receivers and eight input ports. Receiver 0 canconnect to Ports 1 or 5, receiver 1 to Ports 2 or 6, receiver 2 to Ports3 or 7, and receiver 3 to Ports 4 or 8. In the standard configuration,the GEMS MR system has a preamplifier in each input port, except forPorts 1 and 8.

FIGS. 8 and 10 illustrate the interface device, generally designated100, according to its single-receiver version. By its connector 102,interface device 100 is designed to interconnect intracavity probe 1 viaoutput cable 3 to Port 1 of the host MR system 10, which is not equippedwith its own preamplifier. Consequently, interface device 100 includesPIN diode 33 and a preamplifier 101. PIN diode 33 is connected acrossthe input socket 103 of interface device 100 into which plug 35 ofoutput cable 3 plugs. This design choice allows PIN diode 33 to bephysically remote from the intracavity probe 1, thus allowing it to bereused as part of the interface device after the probe 1 is disposed.The preamplifier includes a GASFET 110 and a series resonant inputcircuit 130. The series resonant circuit 130 includes an input capacitorC_(P) and an input inductor L_(P) at the junction of which the gate ofGASFET 110 is also connected. The GASFET has its source connected tobiasing resistor R_(B) and its drain linked to coupling capacitor C_(C)and an RF choke RFC₂. According well known circuit design principles,resistor R_(B) should be selected so that the current flowing throughGASFET 110 will provide a good gain and a low noise figure. RFC₂ allowsDC power to be fed to the drain of GASFET 110 without shorting out theMR RF signals output by preamplifier 101 during the receive cycle of MRsystem 10. A cable trap 115 is preferably employed on the other side ofcapacitor C_(C) to block undesirable cable currents.

When interface device 100 is connected to the MR system via probe cable150 and connector 102, the drain is linked to Port 1 of MR system 10 viacoupling capacitor C_(C) and cable trap 115. The drain is also linked toa DC power source in MR system 10 via the RF choke RFC₂. Bypasscapacitor C_(B2) connects between this RF choke and ground, andtherefore carries any non-DC components to ground. Interface device 100also includes a bypass capacitor C_(B1) and RP choke RFC₁. Bypasscapacitor C_(BI) connects between ground and a biasing line 121 withwhich MR system 10 is able to bias PIN diode 33. C_(B1) thus serves tocarry any non-DC components away from the biasing line and decouplingdiode 33. RFC₁ connects between the anode of PIN diode 33 and bypasscapacitor C_(B1), and thus presents a high impedance to RF frequencieswithout appreciably limiting the flow of the biasing currents. Interfacedevice 100 also preferably includes a preamp protection diode D_(PP) anda bypass capacitor C_(B3). Diode D_(PP) protects preamplifier 101 duringthe transmit cycle of the MR system. Bypass capacitor C_(B3) connectsbetween the anode of the preamp protection diode D_(PP) and ground. RFC₃prevents any RF currents from preamplifier 101 from flowing to MR system10, while allowing the flow of the biasing currents on biasing line 121.

During the transmit cycle, MR system 10 will forward bias diodes D_(D)and D_(PP) via biasing line 121. Situated across the connector 103 ofdevice 100 into which plug 35 of output cable 3 plugs, PIN diode D_(D)will thus decouple intracavity probe 1 as explained above. Meanwhile,preamp protection diode D_(PP) will effectively short circuit the gateof GASFET 110, which prevents the transmitted RF pulse signal fromdamaging preamplifier 101. During the receive cycle, MR system 10 willreverse bias those diodes, effectively turning them off. The seriesresonant circuit 130 will provide optimum impedance to GASFET 110 whencoil loop 2 is operating under loaded conditions. Coupled to the gate ofGASFET 110, the series resonant circuit 130 will provide preamplifier101 with a relatively low input impedance, which serves to broaden thefrequency response of coil loop 2. This broader frequency responseoffsets the fixed tuning scheme, which makes the tuning of coil loop 2far less critical when compared to the probe disclosed in U.S. Pat. Nos.5,476,095 and 5,355,087. More specifically, with coil loop 2 acting as a50 ohm input, series resonant circuit 130 will provide a high impedance(˜1000 to 2000 ohms) to GASFET 110 while appearing as a very lowimpedance (˜1 to 5 ohms) to coil loop 2. This will effectively causecoil loop 2 to decouple somewhat, which broadens its frequency responsewithout sacrificing the signal-to-noise ratio. Along with its seriesresonant input circuit 130, preamplifier 101 will thus provide gain andimpedance matching between the anode of decoupling diode 33 and Port 1so that the MR signals detected by coil loop 2 are passed to Port 1 ofthe MR system with enhanced signal-to-noise ratio.

Interface device 100 also preferably features circuitry 160 to preventthe MR system 10 from performing a scanning procedure when theintracavity probe 1 is not connected to the interface device. Suchcircuitry 160 could create a driver fault within the MR system 10 toprevent it from undertaking a scan when the probe is disconnected. Anaudible alarm or display 161 as part of circuitry 160 through which tonotify medical personnel of such a fault is also preferable.

FIGS. 9 and 11 illustrate the interface device, generally designated200, according to its multiple-receiver version. By its connector 202,interface device 200 is designed to interface not only intracavity probe1 but also a phased array coil system 80 with the Phased Array Port ofthe GEMS 3.0T Signa® MR system. The Phased Array Port is typicallycomposed of four ports (e.g., Ports 2, 4, 5, and 7), all of which areaccessible via a single connector. The prior art Gore® torso array isone such phased array coil system 80 that itself can be plugged via itssingle connector 81 into the Phased Array Port. If the Gore® torso arraywere to be used as coil system 80, coil elements A1 and A2 of FIG. 9would be the two surface coils in the anterior paddle 82, and coilelements P1 and P2 the two surface coils in the posterior paddle 83.Those two paddles each have two coil elements whose leads are routed bymeans of two cables 84,85 to single connector 81. It is by connector 81that the Gore® torso array 80 normally plugs into the Phased Array Portof the host MR system, with each of its four coil elements beinginterconnected with one of the four system ports. Interface device 200,however, when used with intracavity probe 1 and the Gore® torso array,will interface five coil elements (i.e., coil loop 2 and coil elementsA1, A2, P1 and P2) to the four-receiver Phased Array Port of MR system10. Interface device 200 combines the four-coil torso array with thereceive-only endorectal coil 1 to enable high resolution imaging of theprostate along with phased array imaging of the pelvic region.

Interface device 200 includes a probe interface circuit 210 and an arrayinterface circuit 240. Probe interface circuit 210 includes PIN diode 33and a cable trap 211. PIN diode 33 is connected across the input socket203 of device 200 into which plug 35 of output cable 3 plugs. Probecable 213, also referred to herein as circuit length 213, is used tolink the decoupling diode 33—and therethrough coil loop 2 of intracavityprobe 1—with a first port (i.e., Port 7) of MR system 10. Cable trap 211prevents undesired current from flowing on the shield conductor of theprobe cable. As shown in FIG. 9, the circuit length 213 preferably hasan electrical length of n(λ/2), where n is an integer and λ is thewavelength of the operating frequency of the MR system. This makescircuit length 213 effectively appear to have zero electrical length.

The array interface circuit 240 is used to electrically interconnectphased array coil system 80 and MR system 10. It includes first andsecond series resonant networks 242 and 252, two ¼ wavelength networks261 and 262, and a ¼ wavelength combiner 271. Assuming coil system 80takes the form of the Gore® torso array, series resonant network 242will convey the MR signals from anterior coil element A1 to a secondport (i.e., Port 4) of MR system 10. Similarly, the other seriesresonant network 252 will pass the MR signals from anterior coil elementA2 to a third port (i.e., Port 2). As illustrated in FIG. 9, one ¼wavelength network 261 is situated to receive MR signals from posteriorcoil element P1, and the other ¼ wavelength network 262 is configured toreceive MR signals from posterior coil element P2. Preferably of theWilkinson type, the ¼ wavelength combiner 271 is connected to theoutputs of both ¼ wavelength networks 261 and 262. It combines the MRsignals received from those two networks and conveys the resulting MRsignals to a fourth port (i.e., Port 5) of MR system 10.

The first series resonant network 242 includes capacitor C_(R1) and RFchoke RFC₅. Similarly, the second series resonant network 252 includescapacitor C_(R2) and RF choke RFC₆. The values of C_(R1) and C_(R2) areselected so that each capacitor tunes out the inductance inherent in itsrespective circuit path. First and second networks 242 and 252 are thusseries resonant at the operating frequency of MR system 10 (i.e., theyact as if having a length of n(λ/2) where n=0). This enables coil system80 and MR system 10 to operate electrically as if there were no lengthto the networks 242 and 252. In addition, RF choke RFC₅ is disposed inparallel with capacitor C_(R1), as choke RFC₆ is with capacitor C_(R2).This is because, along the circuit paths of series resonant networks 242and 252, MR system 10 will convey biasing signals to the decouplingdiodes in coil system 80 for anterior coil elements A1 and A2. ChokesRFC₅ and RFC₆ allow those biasing signals to pass from Ports 4 and 2 tothose decoupling diodes.

Furthermore, as shown in FIG. 9, the length of the circuit path from theinput for coil element P1 (through network 261 and combiner 271) to Port5 is ideally one-half the operating wavelength (i.e., nλ/2). The samelength applies for the circuit path extending from the input for coilelement P2 to Port 5. Consequently, these circuit paths will effectivelyappear as zero electrical length, which permits the beneficial effectsof the low impedance preamplifier in Ports 5 to be reflected back totheir respective inputs. In addition, MR system 10 conveys biasingsignals to the decoupling diodes for posterior coil elements P1 and P2.An RF choke and related circuitry within combiner 271 and network 261allow biasing signals to pass from Port 5 to the decoupling diode forcoil element P2. An RF choke RFC₇ and related circuitry allow biasingsignals to pass from Port 8 to the decoupling diode for coil element P1.The biasing signals for coil element P1 are sourced from Port 8 so thatit is independent of that for coil element P2.

During the transmit cycle, MR system 10 will forward bias decouplingdiode D_(D) with the decoupling voltage, which is preferablysuperimposed on the signal line of cable 213. Situated across theconnector 203 of device 200 into which plug 35 of output cable 3 plugs,PIN diode D_(D) will thus decouple intracavity probe 1 as explainedabove. MR system 10 will also simultaneously forward bias the decouplingdiodes of the four coil elements A1, A2, P1, and P2 in coil system 80.This will cause those decoupling diodes to short circuit, therebyyielding parallel resonant circuits of high impedance, which willeffectively open circuit the four coil elements of coil system 80. Inthis manner, the host MR system 10 will thus decouple both theintracavity probe 1 and the torso array 80 from the Phased Array Port ofthe MR system. Conversely, during the receive cycle, MR system 10 willreverse bias PIN diode D_(D) of probe 1 and the decoupling diodes ofcoil system 80, effectively turning them off. This will coupleintracavity probe 1 and torso array 80 to the Phased Array Port. Thiswill allow coil loop 2 and coil elements A1, A2, P1 and P2 to detect theMR signals emitted from their respective regions of interest (e.g.,prostate and surrounding abdominal, thoracic and pelvic regions) inresponse to the resonance-inducing RF pulse(s). The MR signals will thenbe routed through interface device 200 in the aforementioned manner andpassed via connector 202 to the Phased Array Port of the host MR system10.

Interface device 200 also preferably features circuitry 280 to preventthe MR system from performing a scanning procedure when the intracavityprobe 1 is not connected to the interface device. Such circuitry 280could include a probe sense line connected to the socket 203 into whichthe plug 35 of intracavity probe 1 plugs. When the probe 1 is connectedto interface device 200 (i.e., plug 35 inserted into socket 203), theprobe sense line would be grounded. Circuitry 280 would then detect theground and pass an appropriate signal to Port 1 to enable the MR systemto begin the scanning procedure. Should the intracavity probe not beconnected to the interface device, circuitry 280 would detect theresulting open circuit and respond by altering the state of Port 1 toprevent the MR system from undertaking the scan. An audible alarm ordisplay 281 as part of circuitry 280 through which to notify theclinician of such a fault is also preferable. Various other methods ofdetermining whether the probe is connected to the interface device are,of course, also contemplated by the present invention.

FIG. 12 illustrates the intracavity probe, and the relevant part of theinterface device corresponding thereto, according to a first alternativeembodiment of the invention. Specifically, FIG. 12 shows coil loop 2 aconnected through output cable 3 a to the decoupling diode D_(D) of theinterface device. Output cable 3 a is unbalanced, with its shieldconductor 31 a connected to junction node 22 a and its center conductor32 a connected to the node on the other side of drive capacitor C_(D1).Unlike the previously disclosed first embodiment, however, output cable3 a has an electrical length of only n(λ/2). This is because thesupplemental length S_(L) has been incorporated within the interfacedevice. This can be accomplished as shown in FIG. 12, for example, byassuring that the electrical length from the input socket to thedecoupling diode D_(D) is equal to S_(L). When output cable 3 a of theprobe is plugged into the interface device, the total electrical lengthfrom the coil loop 2 a to PIN diode D_(D) is then equal to n(λ/2)+S_(L).Although this embodiment puts S_(L) in the interface device rather thanin output cable 3 a, it still allows the intracavity probe and itscorresponding interface device to operate in the same manner as thefirst embodiment of the invention during both the transmit and receivecycles of the MR system.

FIG. 13 illustrates the intracavity probe, and the relevant part of theinterface device corresponding thereto, according to a secondalternative embodiment of the invention. Specifically, FIG. 13 showscoil loop 2 b linked to the decoupling diodes D_(D1) and D_(D2) of theinterface device through a balanced output cable 3 b. At one end ofoutput cable 3 b, the first and second center conductors 32 b and 34 bare connected to the nodes on opposite sides of drive capacitors C_(D1)and C_(D2), respectively. When plugged into the input socket of thecorresponding interface device, output cable 3 b at its proximal end hasits first and second center conductors 32 b and 34 b electrically linkedto the anodes of diodes D_(D1) and D_(D2), respectively, with its shieldconductor 31 b grounded with the cathodes of the two decoupling diodes.Unlike the previously disclosed first embodiment, output cable 3 b hasan electrical length of only n(λ/2), as S_(L) has again beenincorporated within the interface device. Such use of a balanced outputcable 3 b allows better decoupling (e.g., 2×1500Ω across each drivecapacitor) than the unbalanced output cable 3 a used in the firstalternative embodiment.

FIG. 14 illustrates the intracavity probe, and the relevant part of theinterface device corresponding thereto, according to a third alternativeembodiment of the invention. The coil loop 2 c of the probe is linked tothe decoupling diode D_(D) of the interface device through a balancedoutput cable 3 c. Unlike the previous embodiments, coil loop 2 c isconstructed with only one drive capacitor C_(D), with the tuningcapacitor C_(T) positioned within the wire loop diametrically oppositeit. The values of drive capacitor C_(D) and tuning capacitor C_(T) canbe calculated generally according to the foregoing method so as toenable the coil loop 2 c not only to appear as a 50 ohm source to theinterface device but also to resonate at the operating frequency of theMR system. At one end of output cable 3 c, the first and second centerconductors 32 c and 34 c are connected across drive capacitor C_(D).When plugged into the input socket of the interface device, output cable3 c at its proximal end has its first and second conductors 32 c and 34c electrically linked to the anode and cathode, respectively, ofdecoupling diode D_(D) and its shield conductor 31 c grounded with theinterface device. Unlike the previously disclosed first embodiment,output cable 3 c has an electrical length of only n(λ/2), as S_(L) hasagain been incorporated within the interface device.

FIG. 15 illustrates a presently preferred embodiment of the intracavityprobe, generally designated 11. This intracavity probe includes a coilloop and two output cables 3 d and 3 e. The coil loop is preferably thesame as that disclosed in connection with the first embodiment, i.e.,coil loop 2. The output cables 3 d and 3 e, however, are designed tointerconnect coil loop 2 with an interface device in which improvementsnot found in previous embodiments are incorporated. Of equal importanceto this embodiment is the choice of electrical length for the outputcables 3 d and 3 e, as will be explained below. The actual length ofeach output cable extends from coil loop 2 to the end of plug 335. Plug335 thus represents the point at which the output cables 3 d and 3 econnect to the interface device or other external circuit with which theintracavity probe 11 is to be used. As best shown in FIG. 17, the outputcables 3 d and 3 e are preferably housed in a single conduit 30terminating in plug 335.

Relative to the coil loop, output cable 3 d connects by its shieldconductor 31 d to junction node 22 d and by its center conductor 32 d tothe node on the other side of drive capacitor C_(D1). A second cable,output cable 3 e, connects by its shield conductor 31 e to junction node22 d and by its center conductor 32 e to the node on the other side ofdrive capacitor C_(D2). As should be apparent to artisans of ordinaryskill, the manner in which the output cables 3 d and 3 e connect to coilloop 2 dictate that the MR signals conveyed from coil loop 2 to oneoutput cable will be λ/2 radians (180 degrees) out of phase with respectto those conveyed to the other output cable.

In this embodiment, output cables 3 d and 3 e each have an actualelectrical length of S_(L)+n(λ/4), with the value of n being an oddinteger that is typically set to 1 for the reasons noted above. Withthis electrical length, the output cables are designed to be used withan interface device equipped with an electrical length of λ/4 at itsinput, i.e., between socket 303 and the decoupling diodes. As explainedin detail below, this quarter-wavelength electrical length isincorporated into the inputs of both the single-receiver andmultiple-receiver versions of the interface device.

FIGS. 16 and 17 illustrate the interface device, designated 300 in thispresently preferred embodiment, according to its single-receiverversion. As best shown in FIG. 17, interface device 300 by its probecable 350 and connector 302 is designed to interconnect the intracavityprobe (via conduit 30 and plug 335) to Port 1 of the host MR system 10,which is not equipped with its own preamplifier. Consequently, interfacedevice 300 includes a preamplifier 351 along with a phase shiftingnetwork 310 and two PIN diodes 33 a and 33 b.

Phase shifting network 310 features two sub-networks 320 and 330. Thefirst sub-network 320 includes inductor L₁ and capacitor C₁, and thesecond sub-network 330 includes inductor L₂ and capacitor C₂. To enablethe phase shifts (discussed below) to be well tuned, either capacitor C₁or capacitor C₂ or both may be implemented in the form of a variablecapacitor. PIN diode 33 a is connected across the output of firstsub-network 320, and PIN diode 33 b is connected likewise across theoutput of second sub-network 330. Bypass capacitor C₄ connects betweenthe cathode of diode 33 a and ground. Similarly, bypass capacitor C₅connects between the anode of diode 33 b and ground. Bypass capacitorsC₄ and C₅ serve to route non-DC components to ground while enabling thepassage of DC current for the biasing of diodes 33 a and 33 b. ResistorsR₁ and R₂ connect across PIN diodes 33 a and 33 b, respectively. Thevalues of R₁ and R₂ are selected to assure that the voltage drops acrossdiodes 33 a and 33 b are as equal as possible when the diodes arereversed biased during the receive cycle of MR system 10.

Interface device 300 also includes bypass capacitor C_(BI) and RF chokeRFC₁. Bypass capacitor C_(BI) connects between ground and the biasingline 131 with which MR system 10 is able to bias PIN diodes 33 a and 33b. Capacitor C_(BI) serves to carry any non-DC components to ground andthus away from biasing line 131 and decoupling diodes 33 a and 33 b.RFC₁ connects between the anode of PIN diode 33 a and bypass capacitorC_(B1), and thus presents a high impedance to RF frequencies withoutappreciably limiting the flow of the biasing currents.

Preamplifier 351 is preferably the same as that disclosed in connectionwith the first embodiment, i.e., preamplifier 101 of interface device100. Preamplifier 351 includes a GASFET 360 and a series resonant inputcircuit 370. The series resonant circuit 370 includes input capacitorC_(P) and input inductor L_(P) at the junction of which the gate ofGASFET 360 is also connected. The GASFET has its source connected tobiasing resistor R_(B) and its drain linked to coupling capacitor C_(C)and RF choke RFC₂. Among other components, a cable trap 315 ispreferably employed on the other side of capacitor C_(C) to blockundesirable cable RF currents on the outer shield of probe cable 350.

When interface device 300 is connected to the MR system via probe cable350 and connector 302, the drain is linked to Port 1 of MR system 10 viacoupling capacitor C_(C) and cable trap 315. The drain is also linked toa DC power source in MR system 10 via the RF choke RFC₂. Bypasscapacitor C_(B2) connects between this RF choke and ground, andtherefore carries any non-DC components to ground. RFC₂ allows DC powerto be fed to the drain of GASFET 360 without shorting out the MR RFsignals output by preamplifier 351 during the receive cycle of MR system10. Interface device 300 also preferably includes preamp protectiondiode D_(PP) and bypass capacitor C_(B3). PIN diode D_(PP) protectspreamplifier 351 during the transmit cycle of the MR system. Bypasscapacitor C_(B3) connects between the anode of the preamp protectiondiode D_(PP) and ground, and thus blocks DC components from reachingground. RP choke RFC₃ connects at one end to the input side of capacitorC_(P) and at the other end between diode D_(PP) and bypass capacitorC_(B3). RFC₃ thus provides an open RF circuit and a shorted DC circuitto prevent detuning of the tuned circuit implemented via inductor L_(P)and capacitor C_(P).

When the plug 335 of conduit 30 is connected to the socket 303 ofinterface device 300, the output cables 3 d and 3 e of intracavity probe11 connect to the first and second sub-networks 320 and 330,respectively, of interface device 300. It is through sub-networks 320and 330 that a phase shift is implemented so that, during the receivecycle of MR system 10, the MR signals conveyed by output cable 3 d fromcoil loop 2 combine constructively with the MR signals conveyed byoutput cable 3 e from coil loop 2. In this regard, the first and secondsub-networks 320 and 330 are preferably implemented so as to impart apositive λ/4 (+90 degree) phase shift and a negative λ/4 (−90 degree)phase shift on the MR signals received from output cables 3 d and 3 e,respectively. As detailed below, the combined electrical lengthsimparted by the output cables 3 d and 3 e of probe 11 and thesub-networks 320 and 330 of interface device 300 are critical to theproper operation of this preferred embodiment of the invention.

During the receive cycle, MR system 10 will reverse bias the diodes 33a, 33 b and D_(PP) in interface device 300 via biasing line 131,effectively turning them off. By open-circuiting diodes 33 a and 33 b,MR system 10 makes it possible for the MR signals detected by coil loop2 to be conveyed to the input of preamplifier 351 via output cables 3 dand 3 e and sub-networks 320 and 330. Specifically, as noted above, theMR signals applied by coil loop 2 to output cable 3 d are λ/2 radians(180 degrees) out of phase with respect to those applied to output cable3 e. The MR signals carried by output cable 3 d and sub-network 320 thentravel along a first combined electrical length of S_(L)+λ/2, withS_(L)+λ/4 due to output cable 3 d and +λ/4 due to sub-network 320.Simultaneously, the MR signals carried by output cable 3 e andsub-network 330 travel along a second combined electrical length ofS_(L), with S_(L)+λ/4 due to output cable 3 e and −λ/4 due tosub-network 330. As a consequence of this λ/2 radian phase shift broughtabout by network 310, the MR signals carried on these two signal pathsarrive at the junction of series resonant circuit 370 and the gate ofGASFET 360 back in phase relative to each other. This enables MR signalsfrom the two signal paths to combine constructively, with the combinedMR signal driving preamplifier 351 in the same manner as that disclosedabove in connection with preamplifier 101 of interface device 100.Likewise, the low impedance of preamplifier 351 due to series resonantcircuit 370 is reflected back as an inductance to parallel resonate eachof the drive capacitors C_(D1) and C_(D2) of coil loop 2, thusbroadening the frequency response of the coil loop and providing ameasure of preamplifier decoupling as well, all without sacrificing thesignal-to-noise ratio. Along with its series resonant circuit 370,preamplifier 351 thus provides gain and impedance matching between the(MR signals output by intracavity probe 11 appearing at) decouplingdiodes 33 a/33 b and Port 1 so that the MR signals detected by coil loop2 are passed to Port 1 of the MR system with enhanced signal-to-noiseratio.

During the transmit cycle, MR system 10 will forward bias diodes 33 a,33 b and D_(PP) via biasing line 131. By turning on diode D_(PP), MRsystem 10 effectively short circuits the gate of GASFET 360, whichprevents the transmitted RF pulse from damaging preamplifier 351. Byturning on diodes 33 a and 33 b, MR system 10 causes a short-circuit tooccur at the output of each sub-network at an electrical length of λ/4from socket 303, which is the distance between that socket and thedecoupling diodes. Due to the different phase shifts imparted by the twosub-networks 320 and 330, this results in different effective electricallengths between the drive capacitors and their respective decouplingdiodes. Specifically, the electrical length between drive capacitorC_(D1) and the short-circuit at the output of sub-network 320 isS_(L)+λ/2, with S_(L)+λ/4 due to output cable 3 d and +λ/4 due tosub-network 320. Supplement length S_(L), as noted above, inherentlyacts as the inductor L_(D), and ideally has an inductive reactance whosemagnitude is equal to that of the capacitive reactance of capacitorC_(D1). The λ/2 section, however, effectively appears as zero electricallength because it is one-half the operating wavelength. The effectiveelectrical length between capacitor C_(D1) and decoupling diode 33 a isthus S_(L) during the transmit cycle of MR system 10. The forwardbiasing of decoupling diode 33 a thus enables the inherent inductorL_(D) of output cable 3 d and the drive capacitor C_(D1) of coil loop 2to form a parallel resonant circuit. The high impedance of this parallelresonant circuit approximates an open circuit, which effectively openscoil loop 2 around the point where drive capacitor C_(D1) connects toit. Similarly, the electrical length between drive capacitor C_(D2) andthe short-circuit at the output of sub-network 330 is S_(L), withS_(L)+λ/4 due to output cable 3 e and −λ/4 due to sub-network 330.Supplement length S_(L) thus represent the effective electrical lengthbetween capacitor C_(D2) and decoupling diode 33 b during the transmitcycle. When forward biased, decoupling diode 33 b enables the inherentinductor L_(D) of output cable 3 e and the drive capacitor C_(D2) ofcoil loop 2 to form a parallel resonant circuit. The high impedance ofthis parallel resonant circuit approximates an open circuit, whicheffectively opens coil loop 2 around the point where drive capacitorC_(D2) connects to it. In the foregoing manner, the intracavity probe isdecoupled from the transmit field of MR system 10 during the transmitcycle.

The decoupling of intracavity probe 11 can also be viewed from adifferent perspective. As noted above, each of the output cables 3 d and3 e has an electrical length of S_(L)+λ/4. In each output cable, thesupplemental length S_(L), together with the corresponding drivecapacitor of coil loop 2 to which it is connected, act as a sourceimpedance and can be thought of as being connected to a transmissionline whose length is λ/2 (i.e., the sum of the λ/4 section of the outputcable and the λ/4 section of the sub-network to which it is connected).As is well known, standing waves at the resonant frequency points of ashort-circuited transmission line produce an usual effect. In this case,where the length of this transmission line is effectively one half ofthe operating wavelength of the MR system (or some integer multiplethereof), the source will see an impedance identical to that at the endof the transmission line. This technique is sometimes referred to as ahalf-wavelength impedance transformation.

With the intracavity probe 11 connected to interface device 300 or anyother suitable circuit, each output cable 3 d and 3 e is connectedthrough its corresponding subnetwork to one of the short-circuiteddecoupling diodes 33 a and 33 b, each of which effectively serving asthe terminal end of a half-wavelength transmission line. During thetransmit cycle of MR system 10, the S_(L) section of each output cabletogether with the drive capacitor to which it is connected willtherefore see a short circuit at the exact point at which the S_(L)section segues into the ½ wavelength transmission line. Consequently,the inductor L_(D) inherent in the S_(L) section of output cable 3 d andthe drive capacitor C_(D1) of coil loop 2 form a parallel resonantcircuit. The high impedance of this parallel resonant circuitapproximates an open circuit, which effectively opens coil loop 2 aroundthe point where drive capacitor C_(D1) connects to it. Similarly, theinductor L_(D) inherent in the S_(L) section of output cable 3 eeffectively forms an open circuit with the drive capacitor C_(D2) ofcoil loop 2. As a result, when plugged into an interface device of theappropriate design, the intracavity probe 11 will have its coil loop 2effectively decoupled from the transmit field during the transmit cycleof MR system 10.

Moreover, the intracavity probe 11 will decouple from the transmit fieldeven while disconnected from the interface device. As noted above, eachof the output cables 3 d and 3 e has an electrical length of S_(L)+λ/4.In each output cable, the supplemental length S_(L), together with thecorresponding drive capacitor of coil loop 2 to which it is connected,act as a source impedance and can be thought of as being connected to atransmission line whose length is the remaining λ/4 section of theoutput cable. As is well known, standing waves at the resonant frequencypoints of an open-circuited transmission line produce an usual effect.In this case, where the length of this transmission line has been chosento be exactly one quarter of the operating wavelength of the MR system(or some integer multiple thereof), the source will see the exactopposite of the impedance at the end of the transmission line. Thistechnique is sometimes referred to as a quarter-wavelength impedancetransformation.

With the intracavity probe disconnected, each output cable 3 d and 3 ehas an open-circuit at plug 335 (i.e., at the terminal end of thequarter-wavelength transmission line). During the transmit cycle of MRsystem 10, the S_(L) section of each output cable together with thedrive capacitor to which it is connected will therefore see a shortcircuit at the exact point at which the S_(L) section segues into theλ/4 section of the output cable. Consequently, the inductor L_(D)inherent in the S_(L) section of output cable 3 d and the drivecapacitor C_(D1) of coil loop 2 form a parallel resonant circuit. Thehigh impedance of this parallel resonant circuit approximates an opencircuit, which effectively opens coil loop 2 around the point wheredrive capacitor C_(D1) connects to it. Similarly, the inductor L_(D)inherent in the S_(L) section of output cable 3 e effectively forms anopen circuit with the drive capacitor C_(D2) of coil loop 2. As aresult, should medical personnel ever attempt to use the intracavityprobe, according to this presently preferred embodiment, withoutplugging it into a suitable interface device, the coil loop 2 thereinwill nevertheless be effectively decoupled from the transmit field byeach of the two output cables 3 d and 3 e.

FIGS. 18 and 19 illustrate the interface device, designated 400 in thispresently preferred embodiment, according to its multiple-receiverversion. As best shown in FIG. 19, interface device 400 by its connector402 is designed to interface intracavity probe 11 and a supplementalcoil system, such as a torso array, with the receivers of amulti-receiver MR system. The supplemental coil system, for example, maybe implemented in the form of the phased array coil system 80 describedabove in connection with interface device 200. As an extension of thisexample, FIG. 18 shows interface device 400 being used to interconnectthe supplemental coil system and the intracavity probe 11 with thePhased Array Port of the GEMS 3.0T Signa® MR system. Due to thesimilarity between interface devices 200 and 400 in these respects,neither the descriptions of the Phased Array Port and the supplementalcoil system nor the details on how the interface device 400 connects thesupplemental coil system to the Phased Array Port will be repeated here.

Interface device 400 includes a probe interface circuit 401 and an arrayinterface circuit 440. As alluded to above, the array interface circuit440 is preferably the same as the array interface circuit 240 disclosedin connection with interface device 200. It should be apparent, however,that the array interface circuit may be implemented in a variety ofways, the exact manner dependent upon design choice and systemrequirements. Probe interface circuit 401, however, includes a phaseshifting network 410 and two PIN diodes 433 a and 433 b. It should beapparent that the probe interface circuit 401 need not contain apreamplifier if the port in the MR system to which probe interfacecircuit 401 is to be linked is equipped with one. If the port is not soequipped, then a preamplifier and associated circuitry identical orsimilar to that used in interface device 300 may be employed, as shownin FIG. 16.

Phase shifting network 410 is preferably the same as phase shiftingnetwork 310 shown in FIG. 16, as are PIN diodes 433 a and 433 b relativeto diodes 33 a and 33 b of interface device 300. PIN diode 433 a isconnected across the output of first sub-network 420, and PIN diode 433b is connected likewise across the output of second sub-network 430.Like biasing line 131 of interface device 300, biasing line 431 allowsMR system 10 to bias diodes 433 a and 433 b during the transmit andreceive cycles of operation. As a consequence of the λ/2 radian phaseshift brought about by network 410, the MR signals carried on these twosignal paths arrive at node N back in phase relative to each other. NodeN thus represents the point at which the MR signals from the two signalpaths constructively combine. The combined MR signal is what ultimatelydrives the preamplifier in a first port (i.e., Port 7) of MR system 10.

Probe cable 413, also referred to herein as circuit length 413, is usedto link the output of the decoupling diode 433 a and 433 b (i.e., nodeN) with the first port of MR system 10. Cable trap 411 preventsundesired current from flowing on the shield conductor of the probecable. As shown in FIG. 19, the circuit length 413 preferably has anelectrical length of n(λ/2), where n is an integer and λ is thewavelength of the operating frequency of the MR system. This makescircuit length 413 effectively appear to have zero electrical length.Also implemented with the first and second series resonant networks 242and 252 of array interface circuit 240/440, as well as by the circuitpaths between coil elements P1/P2 and Port 5, these zero electricallengths permits the beneficial effects of the low impedancepreamplifiers in the Ports of MR system 10 to be reflected back to theirrespective inputs.

When the plug 335 of conduit 30 is connected to the socket 403 ofinterface device 400, intracavity probe 11 is connected by its outputcables 3 d and 3 e to the first and second sub-networks 420 and 430,respectively, of probe interface circuit 401. Similarly, when interfacedevice 400 is connected to the MR system via connector 402, node N ofprobe interface circuit 401 is linked to Port 7 of MR system 10 viaprobe cable 413.

During the receive cycle, MR system 10 will reverse bias PIN diodes 433a and 433 b in probe interface circuit 401 as well as the decouplingdiodes in array interface circuit 440. With these diodes effectivelyturned off, intracavity probe 11 and torso array 80 will be effectivelycoupled to the Phased Array Port of MR system 10. This will allow thecoil loop in intracavity probe 11 and the coil elements A1, A2, P1 andP2 in coil system 80 to detect the MR signals emitted from theirrespective regions of interest (e.g., prostate and surroundingabdominal, thoracic and pelvic regions) in response to theresonance-inducing RF pulse(s). Specifically, similar to phase shiftingnetwork 310 described above, the MR signals applied out of phase by coilloop 2 to output cables 3 d and 3 e are phase-aligned by sub-networks420 and 430, respectively, with the constructively combined MR signalsdriving the preamplifier in the first port (i.e., Port 7) of MR system10. Coincidentally, similar to array interface circuit 240 describedabove, the MR signals from the A1 and A2 coil elements are routedthrough series resonant networks 442 and 452, respectively, to thesecond and third ports (e.g., Ports 4 and 2) of MR system 10. Likewise,the MR signals from the P1 and P2 coil elements are routed through ¼wavelength networks 461 and 462, respectively, to the fourth port (e.g.,Port 5) of MR system 10 via the ¼ wavelength combiner 471. Althoughinterface device 400 has been illustrated and described herein in thecontext of a 4-receiver MR system, it should be apparent that theinvention is readily adaptable to MR systems having more or even fewerreceivers.

During the transmit cycle, MR system 10 will forward bias PIN diodes 433a and 433 b in probe interface circuit 401. Situated across first andsecond sub-networks 420 and 430, respectively, diodes 433 a and 433 bwill thus decouple intracavity probe 11 as explained above in connectionwith interface device 300. MR system 10 will also simultaneously forwardbias the decoupling diodes of the four coil elements A1, A2, P1, and P2in coil system 80, as described above in connection with interfacedevice 200. This will cause those decoupling diodes to short circuit,thereby yielding parallel resonant circuits of high impedance, whichwill effectively open circuit the four coil elements of coil system 80.In this manner, the host MR system 10 will thus decouple both theintracavity probe 11 and the torso array 80 from the Phased Array Portof the MR system. Finally, as explained supra in, connection withinterface device 300, intracavity probe 11 will also decouple from thetransmit field even while disconnected from the interface device 400.

As should be apparent to persons of ordinary skill in the field ofmagnetic resonance imaging and spectroscopy, the intracavity probe inany of the above embodiments may be constructed with two or more coilloops arranged in a phased array configuration. In addition, two or morecoil loops in a single intracavity probe can be oriented cooperativelyto provide quadrature coverage of the region of interest. The outputcables of such intracavity probes will have to be connected accordinglyto properly link the coil loops to the appropriate interface device.

FIGS. 20A and 20B, for example, illustrate an intracavity probe 12having four coil elements in a phased array configuration in which eachcoil element is critically overlapped by each of the other coilelements. In this particular manifestation, each coil element is shownwith only one drive capacitor. The four output cables are preferablyimplemented with an electrical length of S_(L)+λ/4 in a manner similarto that disclosed in connection with intracavity probe 11.

The presently preferred and alternative embodiments for carrying out theinvention have been set forth in detail according to the Patent Act.Persons of ordinary skill in the art to which this invention pertainsmay nevertheless recognize alternative ways of practicing the inventionwithout departing from the spirit of the following claims. Consequently,all changes and variations which fall within the literal meaning, andrange of equivalency, of the claims are to be embraced within theirscope. Persons of such skill will also recognize that the scope of theinvention is indicated by the following claims rather than by anyparticular example or embodiment discussed in the foregoing description.

Accordingly, to promote the progress of science and the useful arts, theinventor(s) hereby secure by Letters Patent exclusive rights to allsubject matter embraced by the following claims for the time prescribedby the Patent Act.

1. An intracavity probe for use with a magnetic resonance (MR) systemfor obtaining images or spectra of a region of interest within a cavityof a patient, said intracavity probe comprising: (a) a first coil loopfor receiving MR signals from the region of interest, said first coilloop having a plurality of capacitors therein, said plurality ofcapacitors including (i) a first drive capacitor and a second drivecapacitor of approximately equal value serially connected within saidfirst coil loop and at a junction node thereof forming a virtual groundfor electrically balancing and impedance matching said first coil loopand (ii) a tuning capacitor serially connected within said first coilloop diametrically opposite said junction node of said drive capacitorsand having a value selected to resonate said first coil loop at anoperating frequency of said MR system; (b) a first output cableconnected at one end thereof across said first drive capacitor; and (c)a second output cable connected at one end thereof across said seconddrive capacitor; wherein said first and said second output cables (i)each have an electrical length of S_(L)+n(λ/4) wherein S_(L) is asupplemental length whose reactance is of a same magnitude as areactance of said drive capacitor corresponding thereto, n is an oddinteger, and λ is a wavelength of the operating frequency of said MRsystem; and (ii) terminate in a plug therefor for connecting said firstcoil loop to an interface device for said intracavity probe.
 2. Theintracavity probe of claim 1 further comprising: (a) a flexible shafthaving a tip at a distal end thereof, said tip being substantially moreflexible than a remainder of said flexible shaft; (b) an inner balloonconnected to said distal end of said flexible shaft and enclosing saidtip thereof; (c) a non-stretchable material for securing said first coilloop to an anterior surface of said inner balloon; and (d) an outerballoon connected to said distal end of said shaft enclosing both saidinner balloon and said first coil loop secured thereto, said outerballoon for use in positioning said inner balloon within the cavity ofthe patient; such that said non-stretchable material affects inflationof said inner balloon within said outer balloon to enable said firstcoil loop therein to be positioned approximate the region of interestfor optimal reception of said MR signals therefrom.
 3. The intracavityprobe of claim 2 wherein said non-stretchable material focuses inflationof said inner balloon to force a posterior surface of said outer balloonagainst a wall of the cavity then forcing an anterior surface of saidouter balloon against a correspondingly-shaped interior contour of thecavity thereby bringing said first coil loop approximate the region ofinterest for optimal reception of said MR signals therefrom.
 4. Theintracavity probe of claim 2 wherein said outer balloon has an anteriorsurface for conformably fitting to a correspondingly-shaped interiorcontour of the cavity.
 5. The intracavity probe of claim 4 wherein saidouter balloon has a posterior surface, opposite said anterior surfacethereof, comprising at least a pair of undulating folds.
 6. Theintracavity probe of claim 5 further comprising a means for controllinginflation of said inner balloon, said inflation control means beingconnected to said flexible shaft through which a fluid can be conveyedto inflate and deflate said inner balloon.
 7. The intracavity probe ofclaim 6 wherein said inflation control means includes a stop cock forcontrolling passage of the fluid therethrough and release of the fluidtherefrom.
 8. The intracavity probe of claim 6 wherein said flexibleshaft includes: (a) a first lumen for interconnecting said inflationcontrol means and said inner balloon; and (b) a second lumen throughwhich said first and said second output cables are routed from saidfirst coil loop to be made available for connection to said interfacedevice for said intracavity probe.
 9. The intracavity probe of claim 8wherein said inflation control means comprises a compressible inflatorcuff and a tube therewith connected to said first lumen of said flexibleshaft to deliver the gas to said inner balloon upon compression of saidinflator cuff.
 10. The intracavity probe of claim 5 wherein saidnon-stretchable material focuses inflation of said inner balloon toforce said undulating folds of said outer balloon posteriorly against awall of the cavity then forcing said anterior surface of said outerballoon anteriorly against said correspondingly-shaped interior contourof the cavity thereby bringing said first coil loop approximate theregion of interest for optimal reception of said MR signals therefrom.11. The intracavity probe of claim 10 wherein said anterior surface ofsaid outer balloon is saddle-shaped and said correspondingly-shapedinterior contour of the cavity is a rectal prostatic bulge of thepatient.
 12. The intracavity probe of claim 2 wherein said flexibleshaft includes a scale printed on an outer surface thereof.
 13. Theintracavity probe of claim 2 wherein said inner and said outer balloonseach comprise a non-paramagnetic, flame retardant, biocompatible medicalgrade material having low dielectric loss characteristics.
 14. Theintracavity probe of claim 2 further comprising an anti-migration discattachable to said flexible shaft for preventing unwanted movement ofsaid intracavity probe relative to the cavity of the patient.
 15. Theintracavity probe of claim 14 wherein said anti-migration means is adisc having a semi-spherical shape, said disc defining a slot forallowing said disc to be snapped onto said flexible shaft.
 16. Theintracavity probe of claim 2 further comprising a dilator elementslidably mounted on said flexible shaft for dilating an orifice leadingto the cavity to allow easy positioning of said intracavity probe withinthe cavity.
 17. The intracavity probe of claim 2 wherein said outerballoon further comprises lateral indentations therein on which saidfirst coil loop at least partially rests when said outer balloon isuninflated.
 18. The intracavity probe of claim 1 further comprising: (a)a flexible shaft; (b) an inflatable balloon connected to a distal end ofsaid flexible shaft, said inflatable balloon having (i) an anteriorsurface conformable to a correspondingly-shaped interior contour of thecavity and (ii) a posterior surface comprising at least a pair ofundulating folds; and (c) said first coil loop secured within saidinflatable balloon approximate an underside of said anterior surfacethereof; such that when said inflatable balloon is inserted into thecavity and inflated said undulating folds thereof press against a wallof the cavity generally opposite the region of interest thus forcingsaid anterior surface of said inflatable balloon against saidcorrespondingly-shaped interior contour of the cavity thereby bringingsaid first coil loop approximate the region of interest for optimalreception of said MR signals therefrom.
 19. The intracavity probe ofclaim 1 wherein said first and said second drive capacitors each have avalue in a range of approximately 62 pF to 82 pF and said tuningcapacitor has a value in a range of approximately 12 pF to 15 pF. 20.The intracavity probe of claim 1 further comprising a second coil loopidentical in all respects to said first coil loop inclusive of twooutput cables connected thereto, with said first and said second coilloops being arranged in a phased array configuration.
 21. Theintracavity probe of claim 1 further comprising a second coil loopidentical in all respects to said first coil loop inclusive of twooutput cables connected thereto, with said first and said second coilloops being oriented cooperatively to provide quadrature coverage of theregion of interest.
 22. An intracavity probe for use with a magneticresonance (MR) system for obtaining images or spectra of a region ofinterest within a cavity of a patient, said intracavity probecomprising: (a) a first coil loop for receiving MR signals from theregion of interest, said first coil loop having a plurality ofcapacitors serially connected therein, said plurality of capacitorsincluding (i) a drive capacitor for electrically balancing and impedancematching said first coil loop and (ii) a tuning capacitor positioneddiametrically opposite said drive capacitor and having a value selectedto resonate said first coil loop at an operating frequency of said MRsystem; and (b) an output cable connected at one end thereof across saiddrive capacitor, said output cable (i) having an electrical length ofS_(L)+n(λ/4) wherein S_(L) is a supplemental length whose reactance isof a same magnitude as a reactance of said drive capacitor, n is an oddinteger, and λ is a wavelength of the operating frequency of said MRsystem; and (ii) terminating in a plug therefor for connecting saidfirst coil loop to an interface device for said intracavity probe. 23.The intracavity probe of claim 22 further comprising a second coil loopidentical in all respects to said first coil loop inclusive of an outputcable connected thereto, with said first and said second coil loopsbeing arranged in a phased array configuration.
 24. The intracavityprobe of claim 22 further comprising a second coil loop identical in allrespects to said first coil loop inclusive of an output cable connectedthereto, with said first and said second coil loops being orientedcooperatively to provide quadrature coverage of the region of interest.